Recent findings about cannabinoid receptors and relative endogenous lipid ligands raised a large amount of studies by which it has been possible to hypothesize, for this class of compounds, a huge number of therapeutic applications.
The cannabinoid system (composed of endocannabinoids, the enzymes responsible for the biosynthesis and degradation, and the cannabinoid receptors) is mainly, but not exclusively, present in the central and peripheral nervous system, as well as the immune system. Furthermore, the cannabinoid system has been found also in almost all the ocular tissues: ciliated epithelium, ciliary muscle, ciliary body blood vessels, corneal epithelium, anterior segment endothelium, retina, ganglion cells, trabecular meshwork, and Schlemm's canal. It was also found in the synovial membranes of movable joints, the mucous membrane, and the bladder interstitial space.
The first scientific evidence that an N-acyl-ethanolamine, particularly arachidonoyl ethanolamine or anandamide, is able to specifically bond the cannabinoid receptors dates back to 1992. Some N-acyl-ethanolamines (NAE) cannot be strictly considered as endocannabinoids due to their poor affinity for the cannabinoid receptors CB1 and CB2. Anyway, the presence of these NAEs, such as, e.g., palmitoylethanolamine (PEA), would increase the anandamide action by an “entourage effect”.
Palmitoylethanolamine is an endogenous compound that is considered today as a key element in regulating inflammation, itch, and pain. Recent clinical studies have shown the efficiency of PEA in decreasing intraocular pressure in patients undergoing iridotomy. In other clinical studies, the action of PEA in significantly reducing intraocular pressure in patients diagnosed with primary open-angle glaucoma or an intraocular hypertension has been shown. The mechanism whereby PEA promotes the aqueous humor outflow, hence the intraocular pressure lowering has been found only recently. In clinical studies, it has been found that intra-articular levels of PEA dramatically decrease in subject suffering from Rheumatoid Arthritis or Osteoarthritis.
To date, PEA is marketed in 300 mg and 600 mg tablets by the pharmaceutical company Epitech under the trade name Normast and by the pharmaceutical company Medivis under the trade name Visimast. To date, no ophthalmic solutions or injectable solutions for intra-articular use or for intravesical instillation that contain PEA or other NAEs are on the market, since such molecules are not water-soluble.
In fact, the hydrophobic moiety, which is prevalent in the structure of such fatty acid derivatives, makes such compounds practically water-insoluble. The attempts the Applicant carried out to solubilize NAEs in various lipophilic means, such as oils (whether in the form of an oil-in-water or a water-in-oil emulsion), solubilizing polymers or surfactants, failed completely.
Furthermore, the Applicant has assessed the possibility to obtain the solubilization of NAEs by the formation of inclusion complexes in cyclodextrins. However, the solubilization by using various alpha-, beta-, and gamma-cyclodextrins lead to the formation of insoluble or poorly soluble derivatives.